Many technological areas exist in which it is necessary to take precautions in connection with potentially dangerous closing movements in order to protect against persons and objects becoming trapped in the path of the closing movement. Examples of such applications include automatically closing doors, motorized vehicle windows and control guards for motorized presses.
For this purpose it is already known to use so-called safety strips which are in the fact elongate switch strips which can be inserted into the feed circuit for a drive used to bring about a closing movement and which are arranged to interrupt the feed circuit when a certain pressure is exerted thereon.
A disadvantage of the known arrangements of this kind is however that component failure or the break down of electrical conductors can occur which, despite the compression of the safety strip, allow the closing movement to continue so that the absolutely essential protection against entrapment is no longer reliably available.
The principal object underlying the present invention is to provide a circuit arrangement for monitoring and controlling closing and opening movements effected by a reversible drive which ensures that the potentially dangerous closing movement can be made safe by a self-monitoring system.
It is a particular object of the present invention to ensure, in apparatus of the above described kind, that a closing movement can only take place when the apparatus is operating faultlessly and when the safety sensor is not compressed. It is thus intended to provide an absolutely reliable protection against the possibility of an obstacle or a person becoming trapped in the path of the closing movement.
It is a yet further object of the present invention to provide a circuit arrangement of the above described kind which can be constructed witha minimum of technical complexity and expense.